Radio communication system, radio base station, radio communication terminal, radio communication method, and a non-transitory computer readable medium embodying instructions for controlling a device to implement a control method

ABSTRACT

When one base station forms a plurality of cells that use different frequency bands, a handover caused by movement between cells can be successfully performed with a high probability. A radio base station adapted to communicate with a radio communication terminal, comprising: a communication unit which performs a first communication with the radio communication terminal by using a first frequency, wherein the communication unit performs a second communication with the communication terminal by using a second frequency that is different from the first frequency; and a controller which performs a control, of at least one of a first power of the first communication and a second power of the second communication, based on a first power-related quality of the first communication and a second power-related quality of the second communication.

INCORPORATION BY REFERENCE

This application is a continuation application of U.S. application Ser.No. 13/749,045, filed Jan. 24, 2013, which is based upon and claims thebenefit of priority from Japanese patent application No. 2012-012199,filed on Jan. 24, 2012, the disclosures of which are incorporated hereinin their entirety by reference.

BACKGROUND

An exemplary embodiment relates to a radio communication system, a radiobase station, a radio communication terminal, and a radio communicationmethod.

In recent radio communication systems, small base stations called femtobase stations may be newly introduced in addition to existing basestations, to provide femto cell services. Hereinafter, an existing basestation is referred to as a macro base station for the sake ofdifferentiation. Further, in the period of generation changes in radiocommunication systems, it is expected to provide dual-mode base stationsor dual-mode mobile stations corresponding to two old and new radiocommunication systems, and various apparatuses have been proposed(Japanese Unexamined Patent Application Publication No. 2009-290459(Nakata)). For example, when femto base stations are developed at thetiming of the introduction of an LTE (Long Term Evolution) system,interconnectivity to an existing 3G (3rd Generation) system becomesimportant, and a femto cell base station forming both cells of the LTEsystem and the 3G system are required.

Incidentally, the radio communication system typically supports acircuit switching call (CS Call) represented by a voice call and apacket switching call (PS Call). However, since LTE systems at an earlystage of introduction cannot address with a VoIP over LTE (VoIP: Voiceover Internet Protocol) function that is required for the CS Call, whena dual-mode radio communication terminal using the LTE system makes orreceives the CS Call, a CS Fallback function is often supported toforcibly move the terminal from the LTE cell to the 3G cell by ahandover. Further, in this case, a function of performing a handover ofthe radio communication terminal is often supported as well in order toreturn the terminal from the 3G cell to the LTE cell where higher-speedcommunication can be achieved when the radio communication terminal endsthe CS Call.

Meanwhile, when one base station forms the cell of the LTE system andthe cell of the 3G system, coverage areas of the both cells are notnormally matched with each other. One reason for this is that bothsystems use different frequency bands from each other. Even when radioparameters are initially tuned to make the coverage areas of bothsystems match with each other at the stage of the cell design, there isgenerated a difference in coverage areas of both systems since thenumber of users in each cell and radio interference from neighbor cellsare different for each system. In particular, in the case of the femtocell, transmission power of a femto base station is adjusted accordingto propagation loss from the nearest macro base station (Home Node BRadio Frequency (RF) Requirements (FDD) [3GPP TR25.967 v9.0.0]). Thelocation at which the macro base station of the LTE system is installedand the location at which the macro base station of the 3G system isinstalled are not necessarily the same but are normally different. As aresult, the coverage area of the cell of the LTE system and the coveragearea of the cell of the 3G system formed by one base station aredifferent.

In general, since it is necessary to perform complicated cell systemdesigning/evaluation work in order to form the service area of the radiocommunication system, various cell forming methods have been studied.For example, Japanese Unexamined Patent Application Publication No.2006-135673 (Mori et al.) discloses a mobile communication system thatautonomously sets transmission power of a common control channel to forma cell while cooperating with surrounding base stations.

The related arts described above have the following problems.

When a handover caused by movement between cells is performed betweencells having different coverage areas, the handover may be ended infailure in some cases. For example, take an example in which thecoverage area of the source cell is wider than the coverage area of thedestination cell. In this case, when a handover is performed on a radiocommunication terminal which is in the service area in the source cellbut in the outside of the service area in the destination cell, it isexpected that communication is not normally performed in the destinationcell, which results in failure in the handover. In another case in whicha femto base station is used, when a radio communication terminal islocated at a position which is outside the service area in a femto cellwhich is a destination candidate but within the service area in asurrounding macro cell, the radio communication terminal is handed overnot to the candidate cell to which the own femto base station wasoriginally intended to move but to a surrounding macro cell, althoughthe handover itself is not unsuccessful. This prevents a user fromreceiving the benefits of inexpensive communication costs andsignificant improvement in communication data rate since only a limitednumber of users are able to use radio resources exclusively. Thesebenefits are something that the user could have received by continuoususe of the femto cell, not using the macro cell.

Accordingly, when one base station forms a plurality of cells that usedifferent frequency bands, in order to successfully perform a handovercaused by movement between cells with a high probability, it is requiredto appropriately control coverage areas of the plurality of cells.

The cell forming method disclosed by Mori et al. is a method of forminga cell of an own base station in cooperation with surrounding basestations, and cannot be applied to a case in which it is required tocontrol coverage areas of a plurality of cells that use differentfrequency bands when one base station forms the plurality of cells.

SUMMARY

One exemplary object of the exemplary embodiment is to provide a radiocommunication system that is able to control coverage areas of aplurality of cells that use different frequency bands when one basestation forms the plurality of cells.

However, an illustrative embodiment may achieve objectives other thanthose described above. Further, illustrative embodiments are notrequired to achieve the objectives described above, and an illustrativeembodiment may not achieve any of the objectives described above.

A radio communication base station according to one exemplary aspect ofthe exemplary embodiment is a radio base station, adapted to communicatewith a radio communication terminal, comprising: a communication unitwhich performs a first communication with the radio communicationterminal by using a first frequency, wherein the communication unitperforms a second communication with the communication terminal by usinga second frequency that is different from the first frequency; and acontroller which performs a control, of at least one of a first power ofthe first communication and a second power of the second communication,based on a first power-related quality of the first communication and asecond power-related quality of the second communication.

A radio communication base station according to one exemplary aspect ofthe exemplary embodiment is a radio base station, adapted to communicatewith a radio communication terminal, comprising: a communication unitwhich performs a first communication with the radio communicationterminal in a first cell, wherein the communication unit performs asecond communication with the communication terminal in a second cellthat is different from the first cell; and a controller which controls asize of a coverage area, of at least one of a first cell and secondcell, based on a first quality of the first communication and a secondquality of the second communication.

A control method for a radio base station according to another exemplaryaspect of the exemplary embodiment is a control method for a radio basestation, adapted to communicate with a radio communication terminal,comprising: performing a first communication with the radiocommunication terminal by using a first frequency; performing a secondcommunication with the communication terminal by using a secondfrequency that is different from the first frequency; and performing acontrol, of at least one of a first power of the first communication anda second power of the second communication, based on a firstpower-related quality of the first communication and a secondpower-related quality of the second communication.

A non-transitory computer readable medium embodying instructionsaccording to another exemplary aspect of the exemplary embodiment forcontrolling a device to implement a control method for a radio basestation is a non-transitory computer readable medium embodyinginstructions for controlling a device to implement a control method fora radio base station, adapted to communicate with a radio communicationterminal, comprising: performing a first communication with the radiocommunication terminal by using a first frequency; performing a secondcommunication with the communication terminal by using a secondfrequency that is different from the first frequency; and performing acontrol, of at least one of a first power of the first communication anda second power of the second communication, based on a firstpower-related quality of the first communication and a secondpower-related quality of the second communication.

A radio communication system according to another exemplary aspect ofthe exemplary embodiment is a radio base station, adapted to communicatewith a radio communication terminal, comprising: a communication unitwhich receives an first signal by using a first frequency, wherein thecommunication unit transmits an second signal by using a secondfrequency that is different from the first frequency; and a controllerwhich performs a control, of at least one of a first power of the firstsignal and a second power of the second signal, based on a firstpower-related quality of the first signal and a second power-relatedquality of the second signal.

A radio communication system according to another exemplary aspect ofthe exemplary embodiment is a radio base station, adapted to communicatewith a radio communication terminal, comprising: a communication unitwhich performs a first communication with the radio communicationterminal, wherein the communication unit performs a second communicationwith the communication terminal; and a controller which performs acontrol, of at least one of a first power of the first communication anda second power of the second communication, based on a firstpower-related quality of the first communication and a secondpower-related quality of the second communication.

The above and other objects, features and advantages of the presentinvention will become more fully understood from the detaileddescription given hereinbelow and the accompanying drawings which aregiven by way of illustration only, and thus are not to be considered aslimiting the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a schematic configuration of a radiocommunication system according to a first exemplary embodiment of thepresent invention;

FIG. 2 is a block diagram showing a schematic configuration of a radiobase station according to the first exemplary embodiment of the presentinvention;

FIG. 3 is a sequence diagram showing an operation of the radiocommunication system according to the first exemplary embodiment of thepresent invention;

FIG. 4 is a flowchart showing an operation of the radio base stationaccording to the first exemplary embodiment of the present invention;

FIG. 5 is a flowchart showing an operation of a radio base stationaccording to a second exemplary embodiment of the present invention;

FIG. 6 is a block diagram showing a schematic configuration of a radiobase station according to a third exemplary embodiment of the presentinvention;

FIG. 7 is a sequence diagram showing an operation of a radiocommunication system according to the third exemplary embodiment of thepresent invention; and

FIG. 8 is a flowchart showing an operation of the radio base stationaccording to the third exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT

Hereinafter, with reference to the drawings, exemplary embodiments toachieve the present invention will be described.

(1) First Exemplary Embodiment

FIG. 1 shows a schematic configuration of a radio communication systemaccording to a first exemplary embodiment of the present invention.Referring to FIG. 1, the radio communication system according to thisexemplary embodiment includes a radio base station 100 forming a cell110 and a cell 120 that use different frequency bands with each other,and a radio communication terminal 200. The radio base station 100controls transmission power, so as to be able to control coverage areasof the cell 110 and the cell 120. Further, the radio base station 100 isable to instruct the radio communication terminal 200 to measure qualityof radio communication between the radio base station 100 and the radiocommunication terminal 200. Further, the radio base station 100 is ableto receive the results of measuring the quality of the radiocommunication from the radio communication terminal 200.

The radio communication terminal 200 may camp on either the cell 110 orthe cell 120, and is able to communicate with the base station 100 ineither cell. Further, the radio communication terminal 200 is able tomeasure the quality of the radio communication between the radio basestation 100 and the radio communication terminal 200 according to theinstruction from the radio base station 100 to transmit the measurementresults to the radio base station 100.

FIG. 2 is a block diagram showing a schematic configuration of the radiobase station 100. Referring to FIG. 2, the radio base station 100according to this exemplary embodiment includes an antenna unit 101, ameasurement report instruction unit 102, a measurement report processingunit 103, and a transmission power controller 104.

The antenna unit 101 transmits or receives radio waves to communicatewith the radio communication terminal 200.

The measurement report instruction unit 102 instructs the radiocommunication terminal 200 to measure the quality of the radiocommunication between the radio base station 100 and the radiocommunication terminal 200 and to report the measurement results. Theinstruction is transmitted to the radio communication terminal 200 byway of the antenna unit 101 as a message. The message may be separatelytransmitted by the radio base station 100 to the radio communicationterminal 200, or may be a message of system information broadcasted bythe radio base station 100 to the cell 110 and the cell 120.

The measurement report processing unit 103 receives a message to reportthe results of measuring the quality of the radio communication receivedfrom the radio communication terminal 200 by way of the antenna unit101.

The transmission power controller 104 controls transmission power ofradio waves transmitted from the antenna unit 101 based on the resultsof measuring the quality of the radio communication processed by themeasurement report processing unit 103.

FIG. 3 is a sequence diagram showing an operation of the radiocommunication system according to this exemplary embodiment.Hereinafter, with reference to FIG. 3, an operation of the radiocommunication system according to this exemplary embodiment will bedescribed.

In S001, the radio base station 100 instructs the radio communicationterminal 200 to measure and report the quality of the radiocommunication between the radio base station 100 and the radiocommunication terminal 200 in the cell 110 and the cell 120.

Examples of the quality of the radio communication include receptionpower in the radio communication terminal 200. Further, examples of thequality of the radio communication in the case of cells in the LTEsystem include RSRP (Reference Signal Received Power), RSRQ (ReferenceSignal Received Quality), and Pathloss. Examples of the quality of theradio communication in the case of cells in the 3G system includeP-CIPCH (Primary Common Pilot Channel) Ec/No (The received energy perchip divided by the power density in the band.), P-CIPCH (Primary CommonPilot Channel) RSCP (Received Signal Code Power), and Pathloss.

The radio base station 100 may include a measurement/report condition, ameasurement/report target and the like in the instruction. For example,the measurement/report may be performed when the result of measuring thereception power in the cell 110 or the cell 120 does not satisfy apredetermined condition.

In S002, the radio communication terminal 200 measures the quality ofthe radio communication between the radio base station 100 and the radiocommunication terminal 200 in the cell 110 and the cell 120 according tothe measurement condition instructed by the radio base station 100.

In S003, the radio communication terminal 200 transmits the messagereporting the measurement results to the radio base station 100according to the report condition instructed by the radio base station100.

In S004, the radio base station 100 performs transmission power controlin the cell 110 and the cell 120 based on the measurement results thatare received.

While the radio communication terminal 200 reports the measurementresults using the measurement report message in Step S003 in FIG. 3, theradio communication terminal 200 may report the measurement results tothe radio base station 100 with positional information using a LoggedMDT (Minimizing Drive Test).

FIG. 4 is a flowchart showing one example of an operation of the radiobase station 100 according to this exemplary embodiment. Hereinafter,with reference to FIG. 4, an operation of the radio base station 100when cell coverage areas of the cell 110 and the cell 120 aresubstantially matched will be described.

In S011, the radio base station 100 receives the results of measuringthe quality of the radio communication from the radio communicationterminal 200.

In S012, the radio base station 100 determines whether the measurementresults that are received include measurement results in both of thecell 110 and the cell 120.

In S013, when there are both measurement results, the radio base station100 compares the measurement result in the cell 110 with respect to apredetermined reference value of the cell 110 (a first predeterminedreference value). The radio base station 100 compares the measurementresult in the cell 120 with respect to a predetermined reference valueof the cell 120 (a second predetermined reference value). Then the radiobase station 100 compares a relative value A of the measurement resultin the cell 110 with a relative value B of the measurement result in thecell 120. When A is larger than B as a result of the comparison, theradio base station 100 performs such a control as to reduce thetransmission power in the cell 110 by a predetermined control width andto increase the transmission power in the cell 120 by a predeterminedcontrol width. On the contrary, when B is larger than A, the radio basestation 100 performs such a control as to increase the transmissionpower in the cell 110 by a predetermined control width, and to reducethe transmission power in the cell 120 by a predetermined control width.When A is equal to B, the radio base station 100 performs such a controlas to maintain the transmission power in the cell 110 and the cell 120.

When there is a measurement result of only one of the cells, in S014,the radio base station 100 performs such a control as to reduce thetransmission power in the cell with measurement result by apredetermined control width, and to increase the transmission power inthe cell without measurement result by a predetermined control width.

When there are no measurement results in both cells, in S015, the radiobase station 100 performs such a control as to maintain the transmissionpower in both cells.

As stated above, according to this exemplary embodiment, a radio basestation forming the plurality of cells that use different frequencybands is able to control the transmission power so that coverage areasof the plurality of cells form desired areas. Further, a control is madeso that the coverage areas of the plurality of cells are substantiallymatched. Accordingly, when a handover is caused by the radiocommunication terminal moving between cells, even when the radiocommunication terminal exists at an end of a cell, there is a highpossibility that the handover caused by the movement between cells issuccessfully performed. Further, one radio base station is able tocontinue controlling the radio communication terminal. Thus, compared toa case in which a handover across a plurality of different radio basestations is performed, there occurs no instantaneous interruption whenthe radio communication terminal downloads or uploads data, whichimproves the throughput of the communication. Since one radio basestation continues controlling the radio communication terminal, thereare other advantages that it is possible to provide Local NetworkService, to allow a user to receive the benefit of low charge because ofthe continuous use of the femto cell, and to achieve traffic offload andimprovement in the system throughput of a macro cell system due to theradio communication terminal not using the macro cell system.

(2) Second Exemplary Embodiment

A condition to perform the transmission power control may be added tothe first exemplary embodiment. More specifically, in a second exemplaryembodiment, the radio base station 100 performs the transmission powercontrol only when the result obtained by comparing the result ofmeasuring the quality of the radio communication in the cell 110 with apredetermined threshold of the cell 110 and the cell 120 with apredetermined threshold of the cell 120 in S013 of FIG. 4 satisfies apredetermined condition. More specifically, for example, when theresults of measuring the reception power by the radio communicationterminal 100 is smaller than a predetermined thresholds, it is assumedthat the radio communication terminal 100 is at an end of a cell and thetransmission power control is performed. On the contrary, when theresult of measuring the reception power is equal to or larger than thepredetermined thresholds, it is assumed that the radio communicationterminal 100 is at a location such as the center of a cell whichprovides high radio communication quality and the transmission powercontrol is not performed. This is because, the handover caused by themovement between cells is expected to be successful in many cases whenthe radio communication terminal 100 is at the center of the cell,whereas the handover is expected to be unsuccessful in many cases whenthe radio communication terminal 100 is at an end of a cell.

FIG. 5 is a flowchart showing an operation of the radio base station 100according to this exemplary embodiment.

In Step S021, the radio base station 100 compares the result ofmeasuring the quality of the radio communication in each of the cell 110and the cell 120 with each threshold, to determine whether thepredetermined condition is satisfied.

In Step 022, when the measurement result in the cell 110 is equal to orlarger than the predetermined threshold of the cell 110 and themeasurement result in the cell 120 is equal to or larger than thepredetermined threshold of the cell 120, the radio base station 100 doesnot perform the transmission power control.

In Step 023, the radio base station 100 performs the transmission powercontrol when the measurement result in the cell 110 is smaller than thepredetermined threshold of the cell 110 or the measurement result in thecell 120 is smaller than the predetermined threshold of the cell 120.The thresholds may be a predetermined absolute value, or may be arelative value that is determined according to interference power fromneighbor cells. Further, the thresholds may be set by an operation ormay be set according to interference power from neighbor cells.

In this way, according to this exemplary embodiment, it is possible toperform the transmission power control only based on the measurementresults of the radio communication terminal that is located at an end ofa cell, whereby it is possible to perform the transmission power controlwith high accuracy using a smaller amount of processing.

By appropriately specifying the measurement/report condition from theradio base station 100 to the radio communication terminal 200 in S001of FIG. 3, the same effects as the case of performing the operation inFIG. 5 stated above can be obtained. Specifically, such a condition isset that, the radio base station 100 notifies the radio communicationterminal 200 of predetermined thresholds, and the measurement resultsare reported only when the result obtained by comparing the result ofmeasuring the quality of the radio communication with the thresholdssatisfies a predetermined condition. Then, only the radio communicationterminal 200 that satisfies the predetermined condition located at anend of a cell transmits the measurement results to the radio basestation 100, and the radio base station 100 performs the transmissionpower control based on the measurement result.

As stated above, according to this exemplary embodiment, only the radiocommunication terminal that is located at an end of a cell transmits themessage to report the measurement results. Thus, it is possible toreduce the number of messages, the processing load in the radio basestation, and power consumption in the radio communication terminal.

In the exemplary embodiment described above, the radio communicationterminal 200 that is located at an end of a cell is determined bycomparing the result of measuring the quality of the radio communicationwith the threshold. However, when the latitude and the longitude of thelocation of the radio communication terminal 200 are known, it may bedetermined whether the radio communication terminal 200 is located at anend of a cell based on the latitude/longitude information. In this case,as one example, the radio base station 100 is able to cause the radiocommunication terminal 200 equipped with a GPS (Global PositioningSystem) to report the latitude/longitude information of the currentlocation, and compares this information with ranges of predeterminedlatitude and longitude stored in advance that should be regarded as anend of a cell, thereby being able to determine whether the radiocommunication terminal 200 is located at an end of a cell.Alternatively, the radio base station 100 may determine whether theradio communication terminal 200 is located at an end of a cell bycalculating the distance between the radio communication base station100 and the radio communication terminal 200 from the latitude/longitudeinformation of the radio base station 100 and the latitude/longitudeinformation of the radio communication terminal 200 and comparing thedistance with a predetermined thresholds. Further alternatively, it maybe possible to use the latitude/longitude information when the radiocommunication terminal 100 moves between the cell 110 or the cell 120and another cell, or the radio communication terminal 100 moves betweenthe cell 110 and the cell 120 (Cell Reselection or handover) todetermine whether the radio communication terminal 200 is located at anend of a cell. Further alternatively, the latitude/longitude informationwhen the handover caused by the movement between cells is unsuccessfulmay be used to determine whether the radio communication terminal 200 islocated at an end of a cell.

According to this exemplary embodiment, compared to the case in which itis determined whether the radio communication terminal is located at anend of a cell by the reception power measured by the radio communicationterminal, it may be possible to make a determination with higheraccuracy even in such a case in which the radio propagation environmentvaries depending on special circumstances.

In the transmission power control in S013 of FIG. 4 and S023 of FIG. 5,the control width of the transmission power control may be set todifferent values between a case in which a control is made so that themeasurement result is away from the reference value and a case in whicha control is made so that the measurement result is close to thereference value. More specifically, a case will be described as anexample in which the result of measuring the reception power reportedfrom the radio communication terminal 200 to the radio base station 100is −50 (dB), the reference value is −60 (dB), and the value of thecurrent transmission power is 20 (dBm). In this case, when a control ismade so that the measurement result is away from the reference value,i.e., a control is made to increase the transmission power, the value isset to 20+ΔP1 (dBm). On the contrary, when a control is made so that themeasurement result is close to the reference value, i.e., a control ismade to decrease the transmission power, the value is set to 20−ΔP2(dBm). It is possible to set ΔP1 and ΔP2 to different values so thatΔP1<ΔP2, for example.

As described above, in this exemplary embodiment, the control width ofthe transmission power control is adjusted to increase or decrease basedon the result of comparing the measurement result with the referencevalue. In this way, it is possible to make an adjustment so that themeasurement result does not substantially differ from the referencevalue.

(3) Third Exemplary Embodiment

FIG. 6 is a block diagram showing a schematic configuration of a radiobase station 100 according to a third exemplary embodiment of thepresent invention. With reference to FIG. 6, the radio base station 100includes, as is similar to the first exemplary embodiment shown in FIG.2, an antenna unit 101, a measurement report instruction unit 102, ameasurement report processing unit 103, and a transmission powercontroller 104. Since these components are the same to those in thefirst exemplary embodiment, the description thereof will be omitted. InFIG. 6, a measurement result statistical information database 105 isfurther included.

The measurement result statistical information database 105 is adatabase to store the results of measuring the quality of the radiocommunication received from the radio communication terminal 200 asstatistical information.

FIG. 7 is a sequence diagram showing an operation of a radiocommunication system according to this exemplary embodiment.Hereinafter, with reference to FIG. 7, an operation of the radiocommunication system according to this exemplary embodiment will bedescribed.

Since the operations of the radio base station 100 in S101, S102, andS103 are similar to those of the first exemplary embodiment shown inFIG. 3, the description thereof will be omitted.

In S104, the radio base station 100 stores the measurement resultsincluded in the measurement result report message that is received inthe measurement result statistical information database 105. In thisexemplary embodiment, as is different from the first exemplaryembodiment shown in FIG. 3, the radio base station 100 does not performthe transmission power control after receiving the measurement reportmessage in S103, but performs the transmission power control at apredetermined timing as shown below.

FIG. 8 is a flowchart showing one example of an operation of the radiobase station 100 according to this exemplary embodiment. Hereinafter,with reference to FIG. 8, an operation of the radio base station 100according to this exemplary embodiment will be described.

The radio base station 100 performs processing of S112 and the followingprocessing at the timing at which the radio communication terminal 200performs movement between cells in S111. One example of the timing atwhich the movement between cells is performed is when CS Fallback isstarted and ended. Another example of the movement between cells is thatthe radio base station forming a plurality of cells that use differentfrequency bands (not limited to femto cells) moves the radiocommunication terminal from a cell with high communication load to acell with low communication load for the purpose of distributing theload between cells. Further, when attributes of services to be providedare differentiated for each cell, the radio base station may performmovement between cells upon changes in the attributes of the servicesthe radio communication terminal receives. For example, it includes acase in which the moving speed of the user changes when a cell for auser moving at a high speed is separated from a cell for a user movingat a low speed. Further, for example, also when a cell for a user whorequires high communication data rate is separated from a cell for auser who requires constant communication data rate such as Streamingalthough the communication data rate may be low, the radio base stationperforms movement between cells of the radio communication terminalaccording to the change in the communication data rate of the servicethat is used.

In S112, the radio base station 100 refers to the statisticalinformation of the measurement results stored in the measurement resultstatistical information database 105, to compare the sizes of thecoverage areas between the source cell and the destination cell.

When the coverage areas of the cells are substantially equal or thesource cell is smaller (S113) as a result of the comparison in S112, theradio base station 100 need not perform the transmission power control.Then, the radio base station 100 goes to processing of performingmovement between cells (S117).

When the coverage area of the cell in the destination is larger thanthat in the source as a result of comparison in S112, the radio basestation 100 determines in S114 whether the radio communication terminal200 exists at a location at which the movement between cells is expectedto be successful based on the positional information of the currentlocation of the radio communication terminal 200 and the statisticalinformation of the measurement result.

When it is predicted that the movement between cells is successful(S115) as a result of determination in S114, the radio base station 100goes to processing of performing movement between cells (S117).

When it is predicted that the movement between cells is unsuccessful asa result of determination in S114, the radio base station 100temporarily increases the transmission power in the destination cell tothe level at which the movement between cells is expected to besuccessful (S116).

In S117, the radio base station 100 performs the movement between cellsof the radio communication terminal 200.

While not illustrated in FIG. 8, the radio base station 100 may performan operation of gradually decreasing the transmission power for everypredetermined time interval after performing the movement between cells.This is because the transmission power that is temporarily increased inthe destination cell may cause interference with the radio communicationterminal 200 controlled by the radio base station 100 and neighbor radiobase stations.

As described above, according to this exemplary embodiment, thetransmission power control is performed only when the radiocommunication terminal moves between cells, so that it is possible toavoid constant interference with the radio communication terminal orneighbor radio base stations, and to increase the probability that thehandover caused by the radio communication terminal moving between cellsis successfully performed.

It is also possible to periodically perform the transmission powercontrol shown in FIGS. 4 and 5 using the statistical information of themeasurement results stored in S104. More specifically, the radio basestation 100 calculates the average value in a predetermined period fromthe statistical information of the measurement result in the cell 110and the measurement result in the cell 120 periodically, not at thetiming at which the radio base station 100 receives the measurementresults from the radio communication terminal 200 in S011. In S013, theradio base station 100 performs the transmission power control bycomparing the average value that is calculated with the reference value.

As described above, according to this exemplary embodiment, the radiobase station is able to perform transmission power control based notonly on the instantaneous measurement results but also on thestatistical measurement results, whereby it is possible to performtransmission power control with higher accuracy without being affectedby the sudden changes in the radio communication environment.

While the present invention has been specifically described based onpreferable exemplary embodiments, needless to say, the present inventionis not limited to the description above, but may be variously changedwithout departing from the spirit of the present invention.

For example, in the exemplary embodiments stated above, the radio basestation forming the two cells of the cell 110 and the cell 120 has beendescribed as an example. However, the present invention may be appliedto a radio base station that forms three or more cells.

Further, for example, while the cell 110 and the cell 120 use frequencybands different from each other in the above exemplary embodiments, thepresent invention may be applied to other cells as long as the cells areformed by one base station and the coverage areas are substantiallymatched geographically. For example, the present invention may beapplied to cells that employ different wireless access systems: cells ofWiFi (Wireless Fidelity), WiMAX (Worldwide Interoperability forMicrowave Access), CDMA2000 (Code Division Multiple Access 2000), GSM(registered trademark) (Global System for Mobile Communications), inaddition to LTE or 3G cell. The cells employing such different wirelessaccess systems typically use different frequency bands in order to avoidinterference. However, even if the same frequency band is used, thepresent invention can be applied if any measure to remove theinterference is taken and the plurality of cells can coexist. Forexample, the present invention is also applicable to a case in which aplurality of cells are formed so as to allow a radio communicationterminal to move from a cell with high communication load to a cell withlow communication load for the purpose of distributing the load betweencells, or a case in which different cells are used according to theservice attributes (e.g., a case in which a cell for a user moving at ahigh speed is separated from a cell for a user moving at a low speed; acase in which a cell for a user who requires high communication datarate is separated from a cell for a user who requires constantcommunication data rate such as Streaming although the communicationdata rate may be low). Further, the frequency bands used for the cellsare not necessarily fixed. The present invention may be applied to aradio communication system such as cognitive radio which appropriatelychanges the frequency bands to be used in a flexible manner. Further,the cell 110 and the cell 120 may be large cells (several hundred metersor larger in radius) like macro cells whose service is expanded over awide area as a public wireless system, or may be small cells (severaltens of meters or smaller in radius) like femto cells that are mainlyinstalled indoors at home or offices, for example.

As another example, in the above exemplary embodiments, one radio basestation 100 forms both cells of the cell 110 and the cell 120. However,the present invention can also be applied to such a case in which aplurality of radio base stations each form a cell as long as the cellsare geographically close to each other, so that it is possible toappropriately control the coverage areas of the plurality of cells andto obtain the same effects as those in the exemplary embodiments statedabove. In such a case, for example, the radio base station forming thecell 110 and the radio base station forming the cell 120 notify eachother of the measurement results in each of the own radio base stationsand the information regarding the transmission power control using aninterface between the base stations, thereby being able to appropriatelycontrol the transmission power in both cells.

As another example, while the coverage areas of the cells are controlledby controlling the transmission power of the radio base station 100 inthe exemplary embodiments stated above, the coverage areas may becontrolled by controlling the beam tilt angle of the antenna of theradio base station 100.

As another example, in the third exemplary embodiment, as shown in FIG.8, the transmission power in the destination cell is controlled to beincreased when the destination cell is small and the movement betweencells is predicted to be unsuccessful under the condition of the timingat which the radio communication terminal 200 moves between cells (S111,S112, S114, and S116). On the contrary, the transmission power controlmay be performed to decrease the coverage area of the source cell and toincrease the coverage area of the destination cell under the conditionthat the radio communication terminal 200 fails in the movement betweencells, i.e., the radio base station 100 detects the failure of thehandover by the radio communication terminal 200. This is because thecause of the failure of the handover is that, since the coverage area ofthe destination cell is small, the radio communication terminal 100cannot be connected to the destination cell and the radio communicationterminal 100 returns to the source cell or re-connected to another cell.Further, the radio base station 100 may perform the transmission powercontrol every time the failure of the handover is detected, or may countthe number of failures of the handover and perform the transmissionpower control when the number exceeds a predetermined threshold.

According to the present invention, it is possible to provide a radiobase station that controls coverage areas of a plurality of cells thatuse different frequency bands when one base station forms the pluralityof cells. Accordingly, such an effect can be obtained that it ispossible to increase the probability that a handover is successfullyperformed when the handover is performed by a radio communicationterminal moving between cells.

While the invention has been particularly shown and described withreference to exemplary embodiments thereof, the invention is not limitedto these embodiments. It will be understood by those of ordinary skillin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present invention asdefined by the claims.

The whole or part of the exemplary embodiments disclosed above can bedescribed as, but not limited to, the following supplementary notes.

What is claimed is:
 1. A radio base station, adapted to communicate witha radio communication terminal, comprising: a communication unit whichperforms a first communication with the radio communication terminal ina first cell, wherein the first cell is an original cell, wherein thecommunication unit performs a second communication with thecommunication terminal in a second cell that is different from the firstcell, wherein the second cell is a destination cell; and a controllerwhich controls a size of a coverage area, of at least one of a firstcell and second cell, based on a first quality of the firstcommunication and a second quality of the second communication; wherein,when the coverage area of the second cell is smaller than the coveragearea of the first cell, the controller controls the size of at least oneof the first cell and the second cell so that the sizes of the coverageareas of the first cell and the second cell become substantially equal.2. The radio base station according to claim 1, wherein the firstquality and the second quality used in the controller comprise ameasurement result of the radio communication terminal.
 3. The radiobase station according to claim 1, wherein the communication unitreceives the first quality and the second quality from the radiocommunication terminal.
 4. The radio base station according to claim 3,wherein the radio communication terminal is located at an edge of acell.
 5. The radio base station according to claim 1, wherein thecontroller controls the size of the coverage area by setting atransmission power of the radio base station.
 6. The radio base stationaccording to claim 1, wherein the first quality and the second qualityused in the controller comprise at least one of: RSRP (Reference SignalReceived Power), RSRQ (Reference Signal Received Quality), Pathloss,Ec/No (The received energy per chip divided by the power density in theband), and RSCP (Received Signal Code Power).
 7. A control method for aradio base station, adapted to communicate with a radio communicationterminal, comprising: performing a first communication with the radiocommunication terminal in a first cell, wherein the first cell is anoriginal cell; performing a second communication with the communicationterminal in a second cell that is different from the first cell, whereinthe second cell is a destination cell; and controlling a size of acoverage area, of at least one of a first cell and second cell, based ona first quality of the first communication and a second quality of thesecond communication; wherein the controlling comprises: when thecoverage area of the second cell is smaller than the coverage area ofthe first cell, controlling the size of the coverage area of at leastone of the first cell and the second cell so that the sizes of thecoverage areas of the first cell and the second cell becomesubstantially equal to each other.